Zeolite having MFI (structure symbol recommended by International Union for Pure and Applied Chemistry in 1978) type structure, such as ZSM-5 developed by Mobil Oil Corporation since 1972, has been applied extensively as a catalytic material to various hydrocarbon conversion reactions. With the development of new applications, extensive research has been carried out to improve its catalytic performance, besides its synthesis method.
U.S. Pat. No. 3,972,832 discloses a phosphorus-containing zeolite which has been prepared by treating H-ZSM zeolite with a phosphorus-containing compound solution and heating the above-mentioned system. The thus obtained zeolite contains 0.78-4.5 wt. % phosphorus. Said zeolite can be used as a catalyst for the conversion of paraffinic compounds, especially paraffinic hydrocarbons.
U.S. Pat. Nos. 4,374,294, 4,391,739 and 4,399,059 disclose a zeolite catalyst composition containing phosphorus and a metal selected from Group IA and /or Group IIIA (Sc, Y, and RE) of the Periodic Chart of the Elements. The process for preparing said composition comprises impregnating ZSM zeolites with an aqueous diammonium hydrogen phosphate solution, and calcining them at a temperature of 400-700.degree. C. in the presence of 5-100% steam. Said zeolite catalyst exhibits good para-selectivity in the conversion of substituted aromatic compounds.
U.S. Pat. Nos. 4,356,338 and 4,456,780 relate to a method for extending the life of ZSM zeolite catalyst by treating the catalyst with a phosphorus containing compound to deposit 2-15 wt. % phosphorus on said catalyst. Then, said catalyst is calcined at 250-1000.degree. C. in the presence of 5-100% steam for 15 min. to 100 hours. Said method can also be carried out by treating the catalyst first with steam, then with a phosphorus containing compound.
U.S. Pat. No. 4,578,371 discloses a process for the manufacture of a phosphorus containing ZSM-5 zeolite, comprising mixing water, a silicon dioxide source, an alkali metal hydroxide and an aluminum compound, and crystallizing said mixture at a temperature of 95-230.degree. C. without addition of an organic compound, but in the presence of aluminum phosphate.
U.S. Pat. No. 4,605,637 discloses a method for enhancing the catalytic activity of a low acidity zeolite, such as boron-containing ZSM-5 zeolite or ZSM-5 zeolite with a silica/alumina ratio over 70, by contacting said zeolite with an activating solution containing an effective amount of aluminum phosphate at a temperature of 80-370.degree. C.
U.S. Pat. No. 4,791,084 discloses a method for preparing a cracking catalyst by mixing crystalline aluminosilicate particles and alumina particles having been impregnated with a phosphorus component. Said catalyst exhibits better metal poinsoning resistance, higher cracking activity and improved gasoline selectivity in cracking of a heavy oil.
U.S. Pat. No. 5,080,878 relates to a process for reducing the surface activity of ZSM-5 zeolite, by contacting said zeolite with an aqueous fluorosilicate salt, preferably (NH.sub.4).sub.2 SiF.sub.6, to substitute the aluminum atom by silicon atom. Therefore, the catalytic activity of said zeolite is enhanced for a variety of hydrocarbon conversion reactions, in particular, for the oligomerization of olefins to prepare a lube base oil with high viscosity index.
U.S. Pat. No. 5,171,921 discloses a method for producing olefins wherein the catalyst is treated by impregnating ZSM-5 zeolite having a Si/Al ratio of 20-60 with a phosphorus-containing solution so as to render the zeolite to contain 0.1-10 wt. % phosphorus and steam-activating the zeolite at 500-700.degree. C. under a pressure of 1-5 atm. for 1-48 hours. Said steam activation can also be effected by adding 1-50% (mole ratio) steam into hydrocarbon feedstock during the conversion reaction. Said zeolite can be used as an active component in converting olefin and/or paraffin to C.sub.2 -C.sub.5 light olefins.
U.S. Pat. No. 4,650,655 discloses a process for synthesizing ZSM-5 zeolite by using zeolites other than ZSM-5 as seed crystals. It is reported in the patent that ZSM-5 zeolite with a relative crystallinity of 110% is synthesized by using NaY zeolite as seed crystals in the presence of tetrapropyl ammonium bromide and crystallizing at 212.degree. F. (100.degree. C.) for 80 hours. (The relative crystallinity of said zeolite is compared with the zeolite synthesized by using ZSM-5 zeolite as seed crystals)
U.S. Pat. No. 5,232,675 discloses a rare earth-containing high-silica zeolite having penta-sil type structure. The anhydrous composition of said zeolite can be defined by the formula xRE.sub.2 O.sub.3. y Na.sub.2 O. Al.sub.2 O.sub.3 .zSiO.sub.2, wherein x=0.01-0.30, y=0.41.0, and z=20-60. Owing to the 24 times higher normal hexane/cyclohexane adsorption ratio than that of ZSM-5 zeolite, the pore opening of said zeolite is narrower than that of ZSM-5 zeolite. Said zeolite is synthesized by well distributing the seed crystals into a colloid system composed of water glass, an aluminium salt, an inorganic acid, and water, and then crystallizing at 130-200.degree. C., preferably 160-190.degree. C., for 12-60 hours, preferably for 16-30 hours. Said seed crystals are REY, REHY, or REX which contains rare earth elements 2-27% and sodium&lt;7.0%(both based on the weight of oxides). As compared with the conventional ZSM-5 zeolite, said zeolite possesses a higher catalytic activity and better hydrothermal stability.
U.S. Pat. No. 5,380,690 discloses a cracking catalyst for producing lower olefins, wherein a phosphorus and rare earth-containing high silica zeolite having a structure of pentasil (P-ZRP) is used. Said P-ZRP zeolite is prepared by treating the zeolite described in U.S. Pat. No. 5,232,675 with an aluminum phosphate sol, and then activating the zeolite in steam. More specifically, it is prepared by pre-exchanging the zeolite with ammonium ion to reduce its sodium content to a level of less than 0.1 wt. % (based on Na.sub.2 O), then evenly mixing the zeolite with an aluminum phosphate sol according to the weight ratio of aluminum phosphate sol (based on P2O.sub.5): zeolite (dry base)=1:(5-99), followed by calcining the mixture at 300-600.degree. C. for 0.5-6 hours in the presence of 10-100% steam. Said aluminum phosphate sol has a composition of Al.sub.2 O.sub.3 :P.sub.2 O.sub.5 =1:(1-3). The thus-obtained zeolite has a phosphorus content of 2-20 wt. %. Said zeolite catalyst exhibits better hydrothermal stability, improved bottom conversion and higher C.sub.2 -C.sub.5 light olefins yields.
The mesopore of the zeolite plays an important role in improving catalytic performance, especially reaction capacity of large molecules reactions. It has not been reported in the prior art that any MFI type zeolite possesses so large amount of mesopore in the range of 1.0-10 nm.
It is an object of the present invention to provide a highly hydrothermal stable phosphorus containing MFI type zeolite, especially having a large amount of mesopore in the range of 1.0-10 nm. It is a further object of this invention to provide a method for preparing said zeolite.